In the cramped room of a physics laboratory at the heart of Becton Hall, an array of mirrors, lenses and beam-splitters occupy one edge of a table. It may not look like much, but with these materials, Yale physicists have created the world’s first “anti-laser,” a device that completely absorbs light rather than emitting it.
After theoretical physicist Douglas Stone first conceived of an anti-laser two years ago, a group of experimental physicists led by physics professor Hui Cao successfully built and tested the device. The result was a creation brainstormed and constructed at Yale and an invention the physics world had never before considered.
“Basically, it’s something invented here at Yale with no help from anybody outside,” Stone said.
In Stone and Cao’s experiment, published in the Feb. 18 issue of the journal Science, the anti-laser absorbed 99.4 percent of the infrared light — light not quite visible to the human eye — shined at it. In the future, such a device could play a role in wireless communications, high-performance computing and the detection of air pollutants, researchers and experts outside Yale said.
The anti-laser is essentially the opposite of traditional lasers, which emit light after enough energy is pumped into a light-amplifying material called the gain medium. In the anti-laser, the gain medium is replaced by an absorbing medium. Two light beams are shot into opposite ends of the anti-laser and are perfectly absorbed if the device is tuned to the right frequency, said Stone, who is also the chairman of the Applied Physics Department.
The physicists formally call the anti-laser a “coherent perfect absorber,” since it perfectly absorbs light due to the interference of the light beams, Cao said.
“The two beams create an interference pattern, which traps the light infinitely,” Stone said. “Literally forever. It’s this interference, or coherence, of the light that causes it to stay there, and then it has nothing to do but get absorbed.”
Stone said he started thinking about reversing a regular laser when he was trying to explain to a colleague how lasers work in April 2009.
Since it can be confusing to understand how a laser spontaneously emits light, Stone said he encourages people to visualize a laser working backwards. In this case, light beamed at the laser is absorbed, rather than the laser generating light by itself. Mathematically, the processes in a laser should be reversible, Stone said.
“It was literally one of those eureka moments, where you say, ‘Hey, I wonder if you could really do that?’” he said.
Stone took his idea to Cao, and working with postdoctoral students Yi Dong Chong, Wenjie Wan and Heeso Noh, as well as Li Ge GRD ’10, the physicists worked out a theory for the anti-laser and turned it into reality by late 2010.
The anti-laser could have future applications ranging from detecting pollutants in the atmosphere to sending wireless communications, Claire Gmachl, a physicist and professor of electrical engineering at Princeton University said.
When a laser beam is shined on pollutants in the air, the pollutants send a light signal in return, Gmachl said. An anti-laser could be used to absorb this signal, which is very weak and thus normally hard to detect.
“If you have a perfect absorber for a coherent light signal, that’s the best chance you have to actually detect the signal,” Gmachl said.
An anti-laser could also enable wireless communications via laser beams, she added. If information-carrying laser beams were sent from one communication tower to another, an anti-laser on the receiving end could be used to absorb the incoming light and transform it into an electrical signal.
Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy, said he was intrigued by the possibility of combining both a laser and an anti-laser into one device.
“[Such a device would] show the rather exciting and amazing property to simultaneously generate coherent light and to perfectly absorb certain coherent light waves,” Longhi wrote in an e-mail.
Stone and Cao added that the anti-laser could also be useful in a new generation of high-performance computers, which will use optical chips that detect light and change it into electricity. Computer manufacturers are reaching the limit of how small they can make electrical chips, but optical chips could sidestep this problem because light does not interact with itself the way electrons in electrical chips do, Stone said.
Contrary to what people might imagine when they think of a perfect absorber, the anti-laser cannot be used for solar power or stealth technology. Solar panels need to absorb a lot of light, while the anti-laser only absorbs light of a specific frequency. The anti-laser is also unhelpful for defending against laser weapons because it absorbs the light and heat that is sent at it, rather than deflecting the light, he said.
“The whole point of a laser weapon would be to heat you up, burn you up,” Stone said. “Well, this would make it even easier for it to do that.”
The first laser was invented in 1960. Laser is an acronym for “light amplification by stimulated emission of radiation.”